1. Field of the Invention
The present invention relates to an optical communication device and methods of using this device. In particular, the present invention relates to an optical switching apparatus suitable for switching and outputting optical signals received from a plurality of optical transmission lines to other optical transmission lines, and methods for using this apparatus.
2. Prior Art of the Invention
To handle the sudden increase in data traffic through the Internet, etc. and the quickly growing demands for multimedia communication of images, sound and data, much progress has been made to increase the speed and the capacity of the transmission lines and telecommunication network nodes. To achieve a higher transmission speed, optical communication devices and optical fiber transmission lines are generally used to transmit signals between telecommunication network nodes.
In recent years, to handle the ever increasing speed of communication networks and to improve the capacity of communication devices, these communication networks and devices use optical switching apparatuses such as optical cross-connects (hereafter, referred to as OXC) and optical add-drop multiplexing apparatuses (hereafter, referred to as OADM), which implement switching processes such as switching of transmission lines and switching of circuits without converting optical signals to electric signals before processing the signals as in the conventional communication devices.
The OXC or OADM typically includes optical switches as its main components. At present, since a single stage high-capacity optical switch is not commercially available, a high-capacity optical switch is usually implemented through a multi-stage combination of the commercially available low-capacity optical switches such as 2xc3x972 or 8xc3x978 switches. The optical signal power loss and differential loss among the channels of a commercial low-capacity optical switch might reach from several dB to more than ten dB. These losses between the channels might be even larger for a high-capacity switch including a multi-stage combination of the commercially available low-capacity optical switches. Typically, an optical communication system includes optical transmitters and optical receivers before and after optical switches. Since these optical transmitters and receivers have limited optical transmission output powers, sensitivities and dynamic ranges, compensation is generally required for the optical switch loss and differential loss between the channels.
Several methods have been proposed to solve this problem. In xe2x80x9cA Frequency Multiplexed Routing and Selecting Hybrid Switch,xe2x80x9d Denshi Joho Tsushin Gakkai [Electronic Information and Communication Association]/Tsushin Society Taikai [Communication Society Conference (1999)]/B-12-17 (Reference A), a method is disclosed to compensate for the losses by placing optical amplifiers in the middle and/or at the output of the multi-stage optical switches. In xe2x80x9cPower Control in ADM Node Using High-speed Compact-size Optical Spectrum Monitor,xe2x80x9d Denshi Joho Tsushin Gakkai [Electronic Information and Communication Association]/Tsushin Society Taikai [Communication Society Conference (1997)]/B-10-101 (Reference B), it is disclosed that a wavelength-division-multiplexed (WDM) optical signal is first wavelength-demultiplexed by an OADM into an optical signal with multiple wavelengths, and that after controlling the optical amplitude for each of the wavelengths using variable optical attenuators, the signals are again wavelength-division-multiplexed. In this method, the amplitude for each wavelength is controlled based on the results of multiplex signal spectrum monitors after wavelength-division-multiplexing.
Kokai Patent Journal No. HEI 11 [1999]-32010 (Reference C) to the inventor of the present application discloses an OXC containing several optical switches and a few optical amplifiers between the optical switches, wherein the optical signal amplitude is controlled using a configuration wherein the amplification of optical signals is adjusted with the optical amplifiers, which is in turn controlled by the amplitude of the output optical signals.
At present, a high-capacity optical switch is usually realized by combining commercially available low-capacity optical switches in multi-stages. Therefore, it is necessary to appropriately calibrate and install an optical transmission line from the output port of an optical switch at one stage to the input port of another optical switch at the next stage. Thus, maintenance is often required for those transmission lines between the stages, and the optical transmission is interrupted during the maintenance. Further, the interruption may also occur when the high-capacity optical switch is under the normal operation.
A high-capacity switching apparatus, in which optical amplifiers are placed inside or after optical switches, such as the ones disclosed in References A and C, often causes sensitivity degradation of the optical parts on the reception side due to light surges caused by the above described interruption of light. Thus, the configuration disclosed in Reference A or C requires a surge-preventing function in the switching controlling unit of the optical switches and/or the controlling unit of the optical amplifiers. Otherwise, the disclosed high capacity switching apparatus needs to use high performance optical parts such as ones with a wide dynamic range. In addition, to compensate for the optical signals which suffer the power loss in the optical switches, the high-capacity switching apparatus includes optical amplifiers placed after the optical switches. Since the spontaneous emission noise of the optical amplifiers is added to the optical signals with a lowered power, the signal-to-noise ratio of the optical signal may decrease and cause errors in the receiver.
Furthermore, the optical signal received by the input port of an optical switch may take various inner paths before reaching the output port, and the optical switch in each stage is appropriately selected and configured. That is, because the characteristics such as the amplitude loss or the differential loss between the channels of each of the switches in the multi-stage combination is different, the loss between the channels of the optical switches between the input port and the output port will significantly vary depending on the actual configuration of optical switches in the multi-stage combination. Therefore, to offer a high performance large-capacity optical switch, it is desirable to realize compensation for the optical switch loss and the differential loss between channels that have occurred in the chosen optical path for each input/output port. The optical switching apparatuses as disclosed in Reference A or C, however, do not offer the above desired function.
Meanwhile, the OADM as disclosed in Reference B adopts a configuration wherein the spectra of wavelength-division-multiplexed optical signals are monitored and the loss is compensated for each demultiplexed wavelength in the OADM. In this configuration, since the wavelength of each signal to be compensated must be different from one another, the wavelengths and the multiplexing methods of the optical signals used as optical switching apparatuses will be limited. In addition, it is still not compatible with either an optical switch with a flexible configuration wherein the wavelengths monitored by the monitor units correspond to the wavelengths processed by the loss compensation units in a one-to-one fashion. It is desired various connections should be adopted with switching. Alternatively, an optical switch should have a flexible configuration with no restrictions in the wavelength of the optical signals in the multiplexing methods.
It is an objective of certain embodiments of the present invention to provide an optical switching apparatus with improved compensation functions for loss and loss differential between the channels in the optical switching apparatus, and a method of using this apparatus.
It is another objective of certain embodiments of the present invention to provide an optical switching apparatus, wherein the loss and the differential loss between the channels is easily compensated even when a high-capacity optical switch is involved, and a method of using this apparatus.
It is yet another objective of certain embodiments of the present invention to provide a high speed and high capacity optical switching apparatus with a simple configuration and installation procedure, or by including simple hardware and software (or firmware) and a method of using this apparatus. This apparatus has few limitations to the wavelengths of the optical signals or the multiplexing methods, and easily and securely compensates for the loss and differential loss between channels of the optical signals even while the apparatus is in service.
It is still yet another objective of certain embodiments of the present invention to provide an optical switching apparatus and a method of using this apparatus without using special high function parts or control technologies with a simple configuration and installation procedure or by means of simple hardware and software (or firmware). This apparatus has a configuration in which surges would not cause optical parts to be degraded, and compensation for changes in the loss and loss differential among the channels is implemented even while the optical switching apparatus is in service.
Therefore, in one aspect, the present invention relates to an optical switching apparatus. The optical switching apparatus includes an optical switching unit with a plurality of input ports and a plurality output ports, a plurality of input signal adjusting units, and a plurality of output signal monitoring units. The optical switching apparatus further includes a controlling unit connected to the input signal adjusting units, the optical switching unit and the output signal monitoring units. The controlling units select at least one of the input signal adjusting units and at least one of the output signal monitoring units based on the configuration of the optical switching unit and control the amplitude of the optical signals by controlling the selected one input signal adjusting unit based on the feedback from the selected one output signal monitoring unit.
In a preferred embodiment, the present invention relates to an optical switching apparatus including an optical switch with a plurality of input ports and a plurality of output ports, a plurality of optical amplifiers, a plurality of monitor circuits and a controller that supervises and controls the optical switch, the optical amplifiers and the monitor circuits. The optical amplifiers are connected to the respective input ports of the optical switch, and the monitor circuits are connected to the respective output ports of the optical switch. The controller selects one of the monitor circuits according to prescribed rules and obtains the optical power and the differential loss between channels at the output port. The controller selects and controls at least one optical amplifier to according to the setup state of the optical switch. The selected amplifier amplifies the optical signal to be inputted to the input port of the optical switch, and the compensation for the loss and differential loss between channels of the optical switch is made for each input/output port pair of the optical switch. In other words, a pair of an optical amplifier on the input port side of the optical switch and an monitor circuit on the output port side of the optical switch is selected to compensate for the loss and the differential loss between channels of the optical switch for each of its input/output port pair.